A variety of direct current (DC) portable power sources exist to facilitate recharging of portable electronic devices when conventional alternating current (AC) power outlets are unavailable. Such power sources are typically charged from a conventional wall outlet charger, such as a Universal Serial Bus (USB) charger, and then carried by the user (e.g., in a purse, bag, backpack, briefcase, or even a pants or coat pocket) to provide backup charging power for portable electronic devices, such as smartphones, tablet computers, laptop computers, portable gaming systems, or other portable electronic devices.
An electrical block diagram of one conventional portable DC power source 100 (e.g., a portable battery pack) is illustrated in FIG. 1. As illustrated, the DC power source 100 includes, among other things, an input port 101, a battery charging circuit 103, a rechargeable battery 105, a controller 107, one or more discharge circuits 109-110 (two shown for illustration), and one or more output ports 112-113 (two also shown for illustration). The battery charging circuit 103 typically includes battery protection circuitry, including over-voltage protection, over-current protection, and under-voltage protection. The rechargeable battery 105 may include one or more battery cells, such as lithium-ion or lithium-polymer cells, and each discharge circuit 109-110 may convert (step-up or step-down, as appropriate) an output voltage of the battery 105 (e.g., Vbatt) to produce a desired output voltage (Vout) at the associated output port 112-113.
Each discharge circuit 109-110 is typically required to compensate for the difference between the battery voltage and a respective output voltage, which may need to comply with an industry specification, as well as to flatten or linearize the typically non-linear discharge curve of the battery 105. In most situations, the discharge circuit 109-110 is required to increase or boost (step-up) the battery voltage to a desired output voltage level. For example, to comply with the USB specification, the discharge circuit 109-110 may step the battery voltage up from a cell voltage of 2.7-4.4 Volts (V), depending upon the chemistry of the battery cells, to 4.75-5.55 V to produce a USB-compliant output voltage at an output port 112-113, depending on the version of the USB specification (4.75-5.25 V for pre-3.0 specification and 5.0-5.55 V for the USB 3.0 specification).
During operation, the controller 107 senses the battery voltage, the input port 101, and the output port or ports 112-113 (sensing and control lines are shown in dashed form in FIG. 1). When the controller 107 determines that an input voltage (Vin) is present at the input port and the battery voltage is below a threshold (e.g., a top-off threshold), the controller 107 activates the battery charging circuit 103 to enable the battery 103 to receive charging current and a battery charging voltage from an external source applied to the input port 101. When the controller 107 also detects the presence of a load device connected to an output port 112-113 of the power source 100, the controller 107 determines whether the battery 103 is in a state of charge sufficient to supply power to the output port 112-113 to which the load device is connected. For example, the controller 107 may determine whether the battery voltage is greater than a minimum threshold. If the battery 103 is sufficiently charged, the controller 107 may enable the appropriate discharge circuit 109 to convert (e.g., boost (step-up) or buck (step-down), as appropriate) the battery voltage to the appropriate output voltage for the output port 112-113 to which the load device is connected. Therefore, the power source 100 of FIG. 1 provides output power to the output ports 112-113 only after the battery 105 has been charged to a minimum voltage level. The voltage applied to the input port 101 may typically be about 4.0-5.25 V, where the input voltage is received from a conventional USB charger. Additionally, the battery voltage may be 2.7-4.4 V, depending on the battery chemistry. Further, as noted above, the output voltage applied to the output port 112-113 may be about 4.75-5.55 V where the output port 112-113 is USB-compliant. Therefore, according to the configuration of the power source 100 of FIG. 1, the input voltage is dropped down to the battery voltage via linear or switchmode means, and then the battery voltage is boosted or stepped back up to the output voltage. However, such a process can be rather inefficient, resulting in an undesirable loss of energy in the charging circuit 103 and the discharging circuit 109-110 during periods of simultaneous drop-down and boosting functions.
To mitigate such a loss of power, some portable power sources include a bypass circuit to supply power from the input port to an output port when a load device is connected to the output port. A portable power source 200 incorporating such a bypass circuit is illustrated in electrical block diagram form in FIG. 2. As illustrated in the figure, the power source 200 is similar to the power source 100 of FIG. 1 in that the power source 200 includes, among other things, an input port 201, a battery charging circuit 203, a rechargeable battery 205, a controller 207, one or more discharge circuits 209 (one shown for illustration), and one or more output ports 211-212 (two also shown for illustration). The controller 207 monitors the input port 201, the output port or ports 211-212, and the battery voltage or other state-of-charge parameter (sensing and control lines are shown in dashed form in FIG. 2). The controller 207 also controls the activation and deactivation of the battery charging circuit 203 and the discharge circuit 209. Additionally, in contrast to the power source 100 of FIG. 1, the power source 200 of FIG. 2 includes a bypass circuit 214 that, when enabled by the controller 207, supplies the input voltage (Vin) received at the input port 201 to the output port or ports 211-212. The bypass circuit 214 may be implemented using a pair of series-connected, anti-parallel metal-oxide-semiconductor field-effect transistor (MOSFETs) controlled by the controller 207.
As discussed above with respect to FIG. 1, the input voltage may typically be about 4.0-5.25 V, where the external power source applied to the input port 201 is a USB outlet charger. Thus, the input voltage may droop below 4.75 V at times, as is well understood in the art. If the input voltage droops, the voltage at the output port 211-212 (Vout) would be less than 4.75 V, taking into account even the minor voltage drop across the bypass circuit 214. In such a case, the power source's output voltage would not be compliant with the USB specification, which requires an output voltage of 4.75-5.55 V depending on the version of the specification.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated alone or relative to other elements or the elements may be shown in block diagram form to help improve the understanding of the various exemplary embodiments described herein.